Magnetic system for microwave components



Jan. 1, 1957 A. J. sPARLlNG MAGNETIC SYSTEM FOR MICROWAVE COMPONENTS Filed Feb. 4. 1955 INVENTOR.

-MAGNETIC SYSTEM FR MICROWAVE'. "CGMPONENTS Arthur Jodean Sparling, Santa Monica, Calif., assigner to Litton Industries,Inc.,.Beverly Hills, Calif., a corporationpf Delaware AApplication `February@ 1955,1Serial No.1f486,090 1s claims. (crass- 98) "This invention f, relates Eto microwave components of `-thetypevwhereinonewor more specimens of ferrite or similar material are mounted-in =a-waveguide andare t -biasedwith astatic magnetic. tield, and` more particularly Mcludedin this new class of. circuitselements arel load-fiso- `latersfor;gyrators,phasefShifters and attenuators.

Although. the-'motivating concept Vof zthe invention is equally @applicable to thisentirexclass Htif-:circuit elements, `for purt ,ifposes of clarity-and illustrationit-Will `be disclosed with tparticular reference to load isolators.

Basically; aV load isolator,l` or as :it isi sometimesA called, a

i ngyrator; utilizes one "or "more :ferriter specimens` mounted t ina waveguide.v to `achievetoneeway transmission of 'micro- :wave energy. `:-M'ore.specilically,;it 'has' been foundthat by: biasing ta rferritespecimen of predetermined size with .-astaticirnagneticriield of` predetermined magnitude the awaveguide may"be1=made lto.` present.l a: relatively lows'loss Lpath in onefdirectionnand:azrelatively high loss' path in the other direction. Considered in terms ofthe end'result 2 achieved, 'i therefore, the '.devicef is 'analogous to .as crystal rectifier or diode in that it exhibits a low series impedance 1in" the 4forward directionand ahighfseries .impedance in "the .reverse fdirection. For purposes :ofidiscussionthe term insertion loss will be utilized hereinafter todesigrnate'thepower attenuation'of `aload isolator in the for- Vfward ,or lowloss direction, whilej the'term isolation ywill be employed to designate the attenuation :in thei'reverse or high loss direction.

Althoughfload isolators may be utilizedfin .several "different formsofa microwave xsystems, they have been-found 4to be especially applicable toradar systems vfor znon- `reciprocally-isolating-the magnetron or microwave agenerator fromtthe remainder of thefmicrowave circuit. More specifically, if the loadisolatd is coupled tothe magnetron and is 'oriented so. that'. itsv forward direction. is from fthe magnetron to `thetassociated duplexer, thenrsubstantially all of theY output` powerfextracted.` from fthe' magnetron is vtransmitted to the duplexer. fHowever, :any reflections which wouldnormally befreturned to'thema'gnetron `are l iattenuated or absorbed by the load isolator,I therebyxgreab lydecreasing themagnetronssensitivity to load variations `and substantially.Y eliminating for practicalpurposes` what is known to the art as long line effect. .,ln. addition, the utilization of a loadisolator permits .the'degree of coupling-between the resonant system of themagnetron and` its associated output .structure to be` increasedgcon- -sequently, the minimum power output from ia:system.may ibe increased by employing'ail'oad isolator eventthough United States Patent vinsertion loss of. the isolator.

the. magnetron ,output signal. `is .attenuated slightlyfby `the It` willy be recognized. from the foregoing discussion that the insertion loss ofY a load isolator is ideally kept as low., as practical while the det greeof isolationis kept as high as possible.

IInthe prior -art twodilerent types of load isolator. have been developed, the irstI of-which is disclosed in U. S. Patent No. 2,644,930 for Microwave Polarization. Rotation Device .and Coupling.Network,-issued July 7, 1953, toC. H.` Luhrset al. In-this particular type of prior art loadf isolator aferrite` wafer is positioned transversely in a 'cylindrical waveguide-to whosev ends a pair of rectangular waveguides are attached, the transversev axis of one of the rectangular guides being `disposed at an angle of 45 with respect to the transverse axis of the other rectangular waveguide. Incoperation the rferrite is` operable under -the inuenceof amagnetic eld to rotate incident electri- `cal energy through a predeterminedangle which is a function.of` the ferrite size-.and the magnitude of. the :mag-

i netic eld.

The mode of operation of .load isolators of the type disclosed inthe aforementioned patentis described in detail in anarticle` entitled The microwave` gyratorby C.L..Hogan,gpublishe'd in the January 1952 issue'of The Bell System TechnicalJournal. ABriefly stated, this form -offload isolatorrprovides atransmissionsystem which is one halfv wavelength longer', in one direction than in the other, and more specifically, utilizes thewave rotating properties of ferritewhich areV analogous to the phenomenon of Faraday rotationin optics. Accordingly, if the rectangularwaveguides areproperly oriented relative. to

`each other, energy, propagated in one directionis passed while energy transmitted in-` the otherdirection is blocked dueto 4the. orientation ofthe terminal rectangular wave- In general load isolators of the above type havebeen found v-to.have alargenumberof inherent disadvantages which discourage their incorporation inmicrowave systems. Firstly, the units require two waveguide transformers for. interconnecting the cylindrical waveguide with the two rectangular waveguides. Consequently, there is some degree of mismatch which produces undesirable reflections even if the transformers. are -well designed. Secondly, the

.unit ismechanically complex,lengthy and often requires a `twist iny one of the rectangularwaveguides; accordingly the. units are relatively expensive.

Thirdly, the isolation and insertion loss provided by the unit vary greatly over even .a'sma-llportion ofthe microwave spectrum owing principally tothev fact .thatv the angular orientation of the terminal rectangularguides is fixed while the` angular'rotationprovided by the. ferrite `for a Vgiven static field varies -with the frequency of theincident energy. Still. another important .disadvantage of this form of prior art load lisolatoris that its power handling capacity is very limited owing to the fact that the physical disposition of the ferrite in the waveguide `preventsrapid dissipation of the-heat generated .inthe ferrite by the absorption of electrical energy; consequently blowers .or a water jacket must be em- .ployed withthe unit if even a moderateamount of lpower is to be dissipated therein.

vided by a relatively large permanent magnet which contacts the outside periphery of the waveguide walls adjacent the regions whereat the ferrite specimens are affixed to the interior of the waveguide. As in the case of the other prior art load isolator which operates on a principle analogous to Faraday rotation, this newer type of load isolator depends on the fact that the permeability of a ferrite specimen in the presence of a static magnetic field is a non-symmetrical tensor. Consequently, when one or more ferrite specimens are asymmetrically positioned in a waveguide and a static magnetic field is applied transversely to the waveguide, the propagation constant of the modes depends upon the direction of propagation. A more rigorous analysis of this prior art load isolator is set forth on page 816 of the l une 1953 issue of the Journal of Applied Physics in a letter to the editor by Kales et al., entitled A Non-Reciprocal Microwave Component."

Although the latter form of prior art load isolator has succeeded in eliminating many of the disadvantages attendant the original load isolators which employed cylindrical waveguide, it too has several serious disadvantages. Firstly, the permanent magnet required to produce the static magnetic iield is relatively large and bulky, thereby severely restricting the configuration of the magnet and the use of the isolator in many applications where spacing of the microwave components is a critical factor. In addition, the utilization of a relatively large permanent magnet increases the weight of the isolator by several pounds as a consequence of which its use in airborne systems is restricted.

The present invention, on the other hand, provides an improved load isolator which obviates the above and other disadvantages of the load isolators of the prior art. Ac-

cording to the basic concept of the invention, substantially all of the static magnetic iield is channeled through one or more ferrite specimens by a pair of ferrous or ferromagnetic pole pieces built into the waveguide walls for directly incorporating the ferrite specimens in a low reluctance path which includes the magnet that provides the requisite static field.

According to the invention the pole pieces may be flush with both the interior and exterior walls of the waveguide, or may project externally, if necessary, to engage the pole of the magnet. In addition, the thickness of the pole pieces may be varied to effect a transition from a relatively thin ferrite specimen to a relatively thick magnet, thereby further concentrating the magnetic field through the ferrite and substantially eliminating fringing of the static magnetic iield.

More specifically, according to the invention a pair of slots of predetermined dimensions are machined in the sides of the waveguide at the regions where the ferrite specimens are normally aiiiXed to the interior wall of the waveguide. A pair of pole pieces of predetermined size and constructed of magnetic material are then inlaid in the slots so that the innermost ends of the pole pieces are substantially iiush with the interior walls of the waveguide, after which the ferrite specimens are mounted on the inner ends of the pole pieces. The permanent magnet utilized for providing the static field is then brought into engagement with the external ends of the pole pieces, thereby including the ferrite specimens in the low reluctance path of the magnet.

As will be described in more detail hereinbelow, the utilization of pole pieces as herein taught provides load isolators which, for a given insertion loss and isolation, employ magnets which have only of the order of one fourth the size and weight of the magnets heretofore utilized. Consequently, the overall weight of the load isolator may be reduced by a factor of more than one half, while magnets having numerous configurations may be utilized to provide the necessary static field, thereby permitting sufficient iiexibility of design as to allow the load isolator to be utilized in much smaller spaces than formerly required.

In a similar manner, the utilization of a pair of ferrous pole pieces in the other forms of microwave components which employ one or more magnetically influenced ferrite speciments which must be mounted entirely within a waveguide, provides substantially the same advantages. Accordingly, it should be reiterated that the invention is intended to encompass the entire class of microwave devices, including phase Shifters and attenuators, which utilize the ferromagnetic resonance properties of ferrite material.

it is therefore an object of the invention to provide microwave components wherein one or more ferrite specimens mounted entirely in a waveguide are coupled to an external source of magnetic iiux through a low reluctance path.

Another object of the invention is to provide microwave components wherein a static magnetic field is channeled through one or more ferrite specimens mounted entirely in a waveguide by a pair of ferrous pole pieces built into the walls of the waveguide.

Still another object of the invention is to provide microwave load isolators wherein the ferrite specimen is asymmetrically mounted in a rectangular waveguide, the walls of the waveguide adjacent the specimen including built in pole pieces of magnetic material to provide a low reluctance path between the ferrite and an associated permanent magnet.

A still further object of the invention is to provide a relatively inexpensive and light weight microwave load isolator wherein a relatively small magnet is utilized to provide a static magnetic iield for a ferrite specimen mounted in an associated waveguide, the magnet being nterconnected to the ferrite specimen by one or more ferrous pole pieces built into the walls of the wave guide.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

Fig. 1 is an isometric view of one form of load isolator, according to the invention;

Fig. 2 is a cross sectional view of the load isolator of Fig. l;

Fig. 3 is a graph illustrating the isolation provided by an X-band load isolator of the type shown in Figs. l and 2; and

Figs. 4 and 5 are cross sectional views of modilied forms of load isolators constructed in accordance with the principles of the invention.

Referring now to the drawings, wherein like or corresponding parts are designated by like reference characters throughout the several views, there is shown in Fig. 1 a load isolator or gyrator constructed in accordance with the principles of the invention. Basically the load isolator of the invention includes four principal elements, namely, a waveguide section 10 which may include a pair of end flanges 12 and 14, a static magnetic iield generator such as permanent magnet 16 which is formed to generate a magnetic ield transverse to waveguide 10, one or more ferrite specimens, not shown in Fig. 1, which are asymmetrically mounted within the waveguide, and a pair of ferrous pole pieces, such as pole piece 18, which are inlaid or built into the sides of the waveguide for connecting the ferrite to the poles of magnet 16 through a low reluctance path.

As shown in Fig. l magnet 16 and the associated pole pieces extend along the waveguide parallel to its longitudinal axis, the length of the magnet and the pole pieces being slightly larger than the length of the ferrite specimens mounted within the waveguide. Magnet 16 is preferably constructed of Alnico V, although it is clear of i through the ferrite.

. .mt/test2 course, that other good magnetic materials may be uti lized. The pole pieces, on the ,other hand, are preferably composed of a high permeability ferrousor ferromagbe termed ferromagnetic elements, the pole pieces are composed of. a relatively conductive magnetic material, whereas the material of the ferrite specimens should eX- .hibit dielectric characteristics in order to perform the desired function.

-With reference now to Fig. 2 there is shown a cross sectional View of thegload isolatorl of Fig. 1 illustrating the shape and relative positions ofpole piece 18 and its companion pole piece which is designated bythe reference numeral 19, p ole piece 19 being built into the waveguide directly opposite pole piece 18. In the particular embodiment of the invention shown in Fig. 2 the inner ends of the pole pieces are substantially flush with the interior walls of the waveguide while the ferrite utilized for obtaining the unidirectional characteristics of `the device comprises a pair of slab-like specimens 20 and v22. respectively, which are affixed tothe inner sides of the pole pieces. Although the specific isolator illustrated in Fig. 2 `of the drawings is shown to include two ferrite specimens, it will be recognized from the description set forth hereinafter that the invention is equally applicable to isolators -employing only one ferrite specimen, as

shown in the Kales et al. article cited hereinabove.

Ffhe particular shape selected for the pole pieces and their position in the waveguide walls are determined by a number of different factors. More particularly, the position of the pole pieces in the waveguide is deteri mined primarily by the position in which the ferrite specimens are to be mounted. In the particular form of load Y isolators shown in the drawings, the ferrite specimens are mounted below the transverse -aXisl of the 4waveguideapproximately equidistant from the transverse axis and the lower end of the -waveguide Consequently the. pole pieces utilized for mountinglthe ferrite `specimens vare located in the same region.

With respect to the shape of the pole piecesthe `side which surfaces within the waveguide is preferably of approximately the same thickness asthe ferrite specimen which is mountedrthereon'in order to channel or concentrate substantally all of the -static magnetic-field For essentiallythe same reason Aand v in addition, to maintain the reluctance oft-hemagnetic circuit as low as possible, the side of*` the pole piece which engages magnet 16 is preferably'the same size Vas the pole of the magnet. the ferrite `specimens is appreciably` smallerrthan lthe thickness of lthe Vmagnet the .polepieces IareV preferably tapered in order to achieve an optimum balancebetween low reluctancel and low fringing.

Referring again toFig.` 2, it will be notedthat each, of

pole pieces 18 and 19 has a tapered region adjacent -magnet 16 and a region of substantiallyiuniform thickness where the pole pieces extend through thewaveguidegthe advantage of this particular configuration being; that it not only provides the desiredV transition between -therelatively. thin ferrite and the magnet but` that it also simplifies the construction of the isolator. More specifically,

y in `machining the waveguide to receive thepole pieces it is only necessary to mill a pair of slots inoppositesides of the guide, the width of the slots correspondingto thev the mounting of the ferrite specimens on thepolepieces.

Accordingly, when the thickness of -therein. mounted on the pole pieces -with a ysuitable adhesive It has been found that the trst of these operations may be readily performed byfsilver soldering the polepieces tothe waveguide after the pole 4pieces havefbeeninserted Thereafter the ferrite-specimens are l.in turn ticular load isolator are determined primarily in View of the frequency at which the isolator is to be operated, the maximum allowable insertion loss and the minimum isolation to beprovided. In general theratio Iof isolation to insertion loss is a function of the cross sectional area of the ferrite specimens, while the magnitudes of both .the insertion loss andaisolation are substantially linear functions of the length of the ferrite specimens after a predetermined minimum length has kbeen exceeded.

Once-the dimensions of the. ferrite specimens have been determinedthe .static magnetic eld required to produce the desiredisolation concomitant with low insertion loss is best determined. empirically with the aid of an-.electromagnet whichis brought into engagement with therpole pieces. As the strength of the magnetic eld is increased` a point is reached wherea't the isolation proisolation is then designedto provide a static magnetic t eld whose flux densitycorresponds substantially to the pieces `in the load isolator of ,theinventiom fromthefforegoing descriptionthat the degree of isola- :tion provided by a particulanload isolator operating at ux density whereat the isolation wave Was observed to peak.

Consider now the advantages` attendant the use oflpole It is clear a particular frequency is dependent upon the strength of thermagnetic eldthrough the ferrite specimen. Since the size lof the magnetrequired to provide a given iield strength is a directfunctionof the length of the gap in -the magnetic circuit and of` the percentage of the magnetic yux lost through leakage and fringing, it is clear that the pole pieces employedin thelload isolator of the invenrtionmaterially `decrease theisize and weight of themagnet, and in addition, facilitate the use` of magnets `of various configurations.

Referring againrto Fig. 2, the configuration shown for Vmagnet 16 has been found to be especially useful since .it` minimizes the `size yofthe load isolator and in addition,

Longitudinal length lof magnet and pole pieces 2.50 Mean length of magnet from pole to pole 4.63" Thickness of magnet .25" Length of ferrite specimens 2.00

Thickness of specimens (parallel to magnet pole faces) .090"

Height of specimens (perpendicular to magnet pole faces) .065"

It may be recalled that one of the primary advantages of the load isolator of the invention is that the reduced `magnet size permitsthe magnet tohave any of numerous configurations, thereby permitting the isolator to be utilized in systems where the space allotted for a load isolator is relatively small and restricted in shape. With reference now to Figs. 4 and 5, there are shown two other embodiments of load isolators constructed in accordance with the basic teaching of the invention. It will be noted that the configuration of magnet 16 in eachoftnese ern` bodiments of the invention differs from the preferred configuration shown in Fig. 2.

In addition to the distinction in magnet configurations, Figs. 4 and 5 also illustrate two different forms of pole pieces which have been found to function satisfactorily in the load isolator of the invention, while Fig. 4 also illustrates the utilization of pole pieces with a single ferrite specimen 21. As shown in Fig. 4, for example, pole pieces 18 and 19 may be of uniform thickness and need not necessarily be of the same thickness as the ferrite specimens 2t) and 22. The pole pieces in the embodiment shown in Fig. 5, on the other hand, are flush with both the interior and exterior walls of waveguide 10, the gap in magnet 16 being of substantially the same dimension as the overall thickness of the waveguide.

It is readily apparent that numerous for .is of mounting brackets may be utilized in the various embodiments of the invention for maintaining magnet 16 in its position relative to waveguide and pole pieces 13 and 19. It is clear, however, that these brackets should be composed of non-magnetic material, and are preferably constructed of a light weight material such as aluminum, for example. inasmuch as the means utilized for mounting the magnets on the associated waveguide is not material to the invention herein presented, further description thereof is considered unnecessary.

Sumniarizing briefiy the motivating concept herein disclosed, it will be recognized that the present invention provides microwave components wherein one or more ferrite specimens are mounted within a waveguide and are subjected to a static magnetic field provided by an associated permanent magnet, the magnetic field being channeled through the ferrite by a pair of ferrous pole pieces built into the sides of the waveguide, or stated differently, the ferrite being incorporated in a continuous low reluctance path which includes the associated magnet by a pair of pole pieces mounted in the waveguide walls.

It is to be expressly understood, of course, that the accompanying drawings are merely utilized to illustrate the basic concept of the invention as applied to load isolators, and that other modifications iu the invention will be readily apparent to one skilled in the art. For exampie, although the pole pieces are shown to be ush with the interior walls of the waveguide it is obvious that the pole pieces and the contacting ferrite specimens may have conjugally curved surfaces at their interfaces to further concentrate the magnetic field through the ferrite. Accordingly, the scope of the invention is to be limited only by the spirit and scope of the appended claims.

What is claimed as new is:

1. In a microwave component wherein at least one ferrite specimen is mounted entirely in a rectangular waveguide and is subjected to an externally applied static transverse magnetic field, a pair of conductive ferromagnetic pole pieces built into opposite broad sides of the waveguide for mounting the ferrite specimens to channel the magnetic field through the ferrite specimen while maintaining substantially unimpaired the conductivity of said broad sides of the waveguide.

2. In a microwave component which utilizes the ferroresonant properties of a ferrite specimen mounted entirely within a waveguide and positioned in a static magnetic field for altering a microwave signal, the combination comprising: a section of waveguide; at least one conductive ferromagnetic pole piece built into the wall of said waveguide, said pole piece surfacing both within and without said waveguide without appreciably disturbing the electrical characteristics of the waveguide wall of which said pole piece forms a part; a ferrite specimen mounted within said waveguide on said pole piece; and a static magnetic field source having first and second poles and positioned around at least a portion of said waveguide for producing a static magnetic field in the interior of said waveguide, said first pole of said source engaging said pole piece whereby said field is channeled through said ferrite specimen.

3. The combination defined in claim 2 wherein said waveguide is rectangular in cross section and which further includes a second pole piece built into the wall of said waveguide opposite said one pole piece and surfacing both within and without said waveguide, said second pole of said source engaging said second pole piece.

4. The combination defined in claim 3 wherein said ferrite specimen is also mounted on said second pole piece.

5. The combination defined in claim 3 which further includes a second ferrite specimen mounted within said waveguide on said second pole piece.

6. The combination defined in claim 3 wherein said static magnetic field comprises a permanent magnet.

7. An absorption type microwave load isolator comprising: a rectangular waveguide; first and second ferromagnetic pole pieces built into opposite walls of said waveguide, said pole pieces being parallel to each other and to the longitudinal axis of said waveguide and surfacing both within and without said waveguide, said pole pieces substantially preserving the electrical conductivity of the walls of which the form a part; at least one ferrite specimen mounted within said waveguide on at least one of said po-le pieces, the electrical conductivity of said specimen being relatively low compared to that of said pole pieces; and a magnetic field source positioned around at least a portion of said waveguide and having first and second poles, said first and second poles engaging said first and second pole pieces, respectively, for producing a magnetic eld through said ferrite specimen.

8. An absorption type microwave load isolator comprising: a section of rectangular waveguide; first and second conductive ferrous pole pieces inlaid in opposite walls of said waveguide, and surfacing within and without said waveguide, said pole pieces being parallel to each other and to the longitudinal axis of said waveguide and being flush with the interior walls of said waveguide and in electrical contact therewith; first and second ferrite specimens mounted within said waveguide on said first and second pole pieces, respectively; and a permanent magnet, having first and second poles, for providing a transverse magnetic field in said waveguide, said first and second poles engaging said first and second pole pieces without said waveguide whereby said first and second specimens are included in a low reluctance path with said magnet.

9. The microwave load isolator defined in claim 8 wherein said pole pieces are composed of soft iron.

l0. The microwave load isolator defined in claim 8 wherein said pole pieces are built into the broad wall of said waveguide substantially equidistant from its transverse axis and one of its narrow walls.

11. The microwave load isolator defined in claim 10 wherein said pole pieces are fiush with the interior walls of said waveguide and project a predetermined distance beyond the exterior walls of said waveguide, the distance between the projecting ends of said pole pieces being substantially the same as the distance between the poles of said magnet.

12. The microwave load isolator defined in claim l0 wherein said pole pieces are substantially flush with both the interior and exterior walls of said waveguide.

13. In a microwave component which utilizes the ferroresonant properties of a ferrite specimen mounted entirely within a waveguide and positioned in a static mag netic field for altering a microwave signal, the combination comprising: a section of rectangular waveguide; at least one conductive ferromagnetic pole piece built into the wall of said waveguide and in electrical contact therewith; said pole piece surfacing both within and without said waveguide; a ferrite specimen mounted within said waveguide on said pole piece; and a horseshoe magnet positioned adjacent the exterior of said waveguide and having iirst and second poles, said irst pole of said magnet engaging said pole piece whereby said ferrite specimen is included in a low reluctance path with said magnet.

14. An absorption type microwave load isolator comprising: a rectangular waveguide; first and second ferromagnetic pole pieces built into opposite wall-s of said waveguide and being composed of material whose conductivity is of the same -order of magnitude as that of said waveguide, said pole pieces being parallel to each other and to the longitudinal axis of said waveguide and surfacing both within and without said waveguide; at least one ferrite specimen mounted within said waveguide on at least one of said pole pieces; and a static magnetic eld source positioned adjacent said waveguide and having rst and second poles engaging said rst and second pole pieces, respectively, for producing a magnetic lield through said ferrite specimen.

15. In an absorption type load isolator, the combinaguide, said first and second poles engaging said first and second pole pieces without said waveguide whereby said rst and second specimens are included in a low reluctance path with said magnet.

References Cited inthe leof this patent UNITED STATES PATENTS OTHER REFERENCES Kales et al.: A Nonreciprocal Microwave Comtion comprising: a section of rectangular waveguide; iirst 20 ponen, Journal of Applied Physics, vol. 24, No. 6,

and second electrically conductive magnetic pole pieces inlaid in said waveguide, and surfacing within and without said waveguide, said pole pieces being parallel to each other and to the longitudinal axis of said waveguide, first and second ferrite specimens mounted within said waveguide on said rst and second pole pieces, respectively; and a permanent magnet, having first and second poles, for providing a transverse magnetic eld in said wave- 

